The invention relates to a basic frame on the carrying chassis of a truck for receiving units changeable in their orientation and extendable in a plurality of directions, especially for military applications, which has struts extendable in the direction of width and has supports arranged at the end thereof that are vertically lowerable, whereby the extendable struts, which are guided in a transverse hollow beam that is part of the basic frame, are telescopically movable in the longitudinal direction into a desired extended position by a signal transmitted by a control device.
Basic frames equipped with a position detecting system using limit or proximity switches have been proposed, but they do not operate reliably in the presence of electromagnetic radiation. It is however an essential requirement that weapons and signaling devices mounted on a basic frame be fully operational in all circumstances. Even the arrangement of common electromechanical switches is problematic, if such switches are permanently associated with the hollow part of the telescopic structure; the point of switching or detection may be so shifted by the substantial inaccuracies in manufacture of the telescopic frame, which is made up of welded tubes, that the point of switching is no longer accurate.
It is therefore an object of the present invention to provide a reliable detecting or switching means which is not subject to interference in order to ascertain the end position of the telescopically extendable struts with their terminally mounted supports.